Warplanes of Japan: Aichi M6A1 Seiran

Aichi M6A1 Seiran

(USN Photo)

Aichi M6A1 Seiran (Clear Sky Storm or Mist on a Fair Day) being examined by USN sailors at Nagoya, Japan, Sep 1945.

The Aichi M6A Seiran (晴嵐, "Clear Sky Storm") is a submarine-launched attackfloatplane designed for the Imperial Japanese Navy during World War II. It wasintended to operate from I-400 class submarines whose original mission was toconduct aerial attacks against the United States.

From the late 1920s, the Imperial Japanese Navy had developed a doctrine ofoperating floatplanes from submarines to search for targets.[2] In December1941, Commander-in-Chief of the Japanese Combined Fleet, Admiral IsorokuYamamoto, proposed constructing a large fleet of submarine aircraft carriers(also designated STo or sen-toku — special submarine) whose purpose was tomount aerial attacks against American coastal cities. The submarines wouldsurface to launch their aircraft by catapult, submerge to avoid detection, thensurface again to retrieve the aircrews who would ditch their planes nearby. ByJune 1942, the plan was to build a fleet of eighteen such submarines. This waslater cut to nine, then five and finally just three as Japan's wartime fortunesdeclined.

To equip the submarine aircraft carriers, the Imperial Japanese Navy AirService requested that Aichi design a folding attack aircraft with a range of1,500 km (810 nmi) and a speed of 555 km/h (300 kn). Aichi was alreadymanufacturing under license, the D4Y1 Suisei (Judy), a relatively smallsingle-engined carrier dive bomber with exceptionally clean lines and highperformance. Detailed engineering studies commenced in an effort to modify theSuisei for use aboard the I-400 submarines but the difficulties in doing sowere eventually judged insurmountable and a completely new design wasinitiated.

Aichi's final design, designated AM-24 by Aichi and given the militarydesignation M6A1, was a two-seat, low-winged monoplane powered by a 1,050 kW(1,410 hp) Aichi AE1P Atsuta 30 engine (a licence-built copy of theDaimler-Benz DB 601 liquid-cooled V12 engine). The original specificationdispensed with a traditional undercarriage but it was later decided to fit theaircraft with detachable twin floats to increase its versatility. If conditionspermitted, these would allow the aircraft to land next to the submarine, berecovered by crane and then re-used. The floats could be jettisoned in flightto increase performance or left off altogether for one-way missions.[6] TheSeiran's wings rotated 90 degrees and folded hydraulically against theaircraft's fuselage (with the tail also folding down) to allow for storagewithin the submarine's 3.5 m (11 ft) diameter cylindrical hangar. Armament wasa single 850 kg (1,870 lb) torpedo or an equivalent weight in bombs. One 13 mm(0.51 in) Type 2 machine gun was mounted on a flexible mounting for use by theobserver.

As finalized, each I-400 class submarine had an enlarged watertight hangarcapable of accommodating up to three M6A1s. The Seirans were to be launchedfrom a 26 m (85 ft) compressed-air catapult mounted on the forward deck. Awell-trained crew of four men could roll a Seiran out of its hangar on acollapsible catapult carriage, attach the plane's pontoons and have it readiedfor flight in approximately 7 minutes.

In order to shorten the launching process and eliminate the need fortime-consuming engine warm-ups, the Seirans were to be catapulted from a coldstart. This necessitated heating the engine oil for each plane to approximately60 °C (140 °F) in a separate chamber and pumping it, as well as hot water,through the engine just prior to launch while the planes were still in thehangar. In this way, the aircraft's engine would be at or near normal operatingtemperature immediately upon getting airborne. The idea was borrowed from theGermans who planned on using a similar launch method for the aircraft of theirunfinished carrier Graf Zeppelin.

The first of eight prototype Seirans was completed in October 1943, commencingflight testing in November that year. A problem with overbalance of theauxiliary wings was eventually solved by raising the height of the tail fin.Further testing was sufficiently successful for production to start in early1944. In order to aid pilot conversion to the Seiran, two examples of a landbased trainer version fitted with a retractable undercarriage were built. Thesewere given the designation M6A1-K Nanzan ("Southern Mountain").Besides the difference in landing gear, the vertical stabilizer's top portion,which was foldable on the Seiran, was removed.

The first production examples of the Seiran were completed in October 1944.Deliveries were slowed by an earthquake near Nagoya on 7 December 1944, and byan American air-raid on 12 March 1945. Construction of the STo submarines wasstopped in March 1945, after two submarine aircraft carriers had been completedand a third finished as a fuel tanker. These were supplemented by two smallerType AM submarines, originally designed as command submarines carryingreconnaissance floatplanes, but capable of carrying two Seirans. Owing to thereduced carrier submarine force, production of the Seiran was halted, with atotal of 28 completed (including the prototypes and the M6A1-Ks).

The new submarines and aircraft were assigned to the 1st Submarine Flotilla,comprising the two STo submarines, the I-400 and the flagship I-401, eachcarrying three Seirans together with two type AMs, the I-13 and I-14. The 1stSubmarine Flotilla commenced training with the Seirans in January 1945, thecrews gradually learning how to handle the submarines and aircraft. Launchingall three Seirans took longer than expected: 30 minutes if floats were fitted,although this could be reduced to 14.5 minutes if the floats were not used.

The first mission of the Seiran squadron, which was named the ShinryuuTokubetsukougeki-tai (神龍特別攻撃隊, meaning "God-Dragon SpecialAttack Squad") [17] was to be a surprise air strike on the Gatun locks ofthe Panama Canal, to cut the main supply line for US forces in the Pacific.When the force was finally ready to set off on their mission against Panama,Japan's increasingly desperate situation led to a change in plan, with thetarget for the attack, called Operation Hikari (Splendour), being switched tothe American base at Ulithi Atoll where forces, including aircraft carriers,were massing in preparation for attacks on the Japanese Home Islands. Theflotilla departed Japan on 23 July 1945 and proceeded towards Ulithi. On 16August, the flagship I-401 received a radio message from headquarters,informing them of Japan's surrender and ordering them to return toJapan.[18][19][20] All six Seirans on board the two submarines, having beendisguised for the operation as American planes in violation of the laws ofwar,[10] were catapulted into the sea with their wings and stabilizers folded(for the I-401) or pushed overboard (for the I-400) to prevent capture.

Variants

A M6A1-K Nanzan
M6A1
Prototypes powered by Atsuta 30 or 31 1,044 kW (1,400 hp) engine and removablefloats, 8 built.
M6A1 Seiran (Shisei-Seiran)
Special Attack Bomber, 18 built.[22]
M6A1-K Nanzan (南山, "SouthMountain")('Shisei-Seiran Kai)
Prototypes of training version, retractable wheeled landing gear, 2 built.
M6A2
Prototype modification, powered by Mitsubishi Kinsei MK8P 62 1,163 kW (1,560hp) engine, 1 built. (Wikipedia)

(IJNAAF Photos)

Aichi M6A1 Seiran (Clear Sky Storm or Mist on a Fair Day) Japan, c1944.

(SDASM Archive Photo)

Aichi M6A1 Seiran.

(NASM Photo)

(Author Photos)

Aichi M6A1 Seiran (Clear Sky Storm or Mist on a Fair Day) on display in the Paul E. Garber facility, Suitland, Maryland before being moved to the Steven F. Udvar-Hazy Center.  

(Eric Salard Photo)

Aichi M6A1 Seiran (Clear Sky Storm or Mist on a Fair Day) on display in the Steven F. Udvar-Hazy Center.

The Aichi M6A Seiran (Clear Sky Storm or Mist on a Fair Day) was a submarine-launched attack floatplane.  It was intended to operate from I-400 class submarines whose original mission was to conduct aerial attacks against the United States.  A single M6A1 has been preserved and resides in the Udvar-Hazy Center of the Smithsonian's National Air and Space Museum.  It is located in the Washington, DC suburb of Chantilly, Virginia near Dulles International Airport.  The Seiran was surrendered to an American occupation contingent by Lt Kazuo Akatsuka of the Imperial Japanese Navy, who ferried it from Fukuyama to Yokosuka.  The US Navy donated it to the Smithsonian Institution in November 1962.  Restoration work on the Seiran began in June 1989 and was completed in February 2000.  There does not appear to be an FE or T2 number for this aircraft.

(HawkeyeUK Photo)

Aichi M6A1 Seiran (Clear Sky Storm or Mist on a Fair Day) on display in the Steven F. Udvar-Hazy Center.

(USN Photos)

Aichi M6A1-K Nanzan.  

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